ubi_tools: extend supported volume size to 64 bits; shrink code

ubi_tools: extend supported volume size to 64 bits; shrink code

function                                             old     new   delta
get_num_from_file                                      -      90     +90
packed_usage                                       29395   29377     -18
ubi_tools_main                                      1349    1141    -208
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/2 up/down: 90/-226)          Total: -136 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>